It is well known in the field of blast furnace design to use refractory materials, such as carbon blocks, in building the bottom-lining of the hearth. Since it contains liquid hot metal, the working conditions of the hearth lining are severe in view of high temperature, mechanical abrasion, chemical attack and of infiltration of liquid hot metal. The current trend toward increasing the production rate of blast furnaces renders the working conditions even more severe. In order to increase the working life of the bottom lining especially, a known solution consists in providing an uppermost layer of ceramic material, such as fired bricks, e.g. andalusite bricks with mullite bond, on top of the main refractory layer, which is typically made of thermally conductive carbon refractory blocks.
The upper layer of ceramic material, sometimes called the ceramic pad, enhances among others the beneficial effect of the bottom cooling system. The bottom cooling system cools the thermally conductive refractory elements of the bottom lining to achieve a thermal equilibrium, in which the solidification isotherm (the “freeze level”), that is the level at which pig iron solidifies, is located as high as possible in the bottom lining. The ultimate goal is to ensure that any molten cast iron, which would eventually migrate down into the bottom lining, would be solidified at a location as high as possible, preferably at the level of the uppermost ceramic portion (the ceramic pad) if any. Providing an additional thermally insulating barrier of ceramic elements between the bath and the main refractory of the bottom obviously contributes to achieving the latter aim. It can be easily understood that the thermal conductivity of the ceramic layer should be as low as possible. Consequently, the ceramic top layer's main function is to protect the underneath refractories against erosion and generally to reduce their working temperature, which is known to reduce wear.
It has recently been observed however that the approach of providing an uppermost layer of protective ceramic refractory still presents shortcomings. In fact, besides unavoidable long-term wear-off of the ceramic layer, it has been observed that the solidification isotherm starts to progressively descend down into the carbon part of the bottom lining even when no significant reduction in the thickness of the ceramic layer has yet occurred.